1. Field of the Invention
This invention relates to circuits, and more particularly to a circuit which may be fabricated with each of a plurality of radiant energy detectors to form a sensor with improved characteristics, for example, subtraction of all background fluctuations below a predetermined rate in an infrared sensor.
2. Description of the Prior Art
One of the fundamental differences between infrared sensing and visual sensing is that in infrared sensing, a very large background photon flux exists which can be many thousands of times the desired signal to be detected. Because this large background signal may have both spacial and slowly varying temporal variations, it is necessary, particularly in staring space infrared sensors, to have some means subtracting of the fixed background signal at a per pixel level so that the scanning charge coupled device (CCD) which may be used to interface with the detector or an array of detectors is not overloaded. Overloading may occur, for example, in a CCD shift register used to accumulate charge from a number of detectors over a period of time during time delay integration (TDI). The subtraction of fixed or steady background signals in infrared sensors occurs automatically in pyroelectric vidicon type detectors, such as those which use tri-glycine sulfate (TGS) targets. In these pyroelectric targets, only the change in charge appears on the target lead.
In contrast, in present monolithic extrinsic silicon detector arrays, no inherent physical mechanism exists for subtracting off the background charge.
It is therefore desirable to provide a circuit for shunting portions of signals below a predetermined frequency and for providing the remaining signal above the predetermined frequency which may be utilized with a detector to provide fixed background subtraction.
It is further desirable to provide a circuit for fixed background subtraction which will not introduce excess noise, utilizes simple circuit elements and can be fabricated within the pixel size of a detector such as 0.05.times.0.05 mm.
It is further desirable to provide a circuit for fixed background subtraction which can operate at the lowest flux level anticipated for the detector.
It is further desirable to provide a circuit for fixed background subtraction which includes means for adjusting the roll-off frequency between cut-off and acceptance of the signal for transfer to the charge coupled output device such as from a tenth of a Hertz and below and at very low photon flux levels of approximately 10.sup.12 photons/cm.sup.2 sec.
It is further desirable to provide a circuit for each detector of an array of detectors wherein the circuits may be fabricated as an integral part of the array.